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Alarms Reference

This reference guide covers expression syntax, best practices, and troubleshooting for alarms.

Expression Syntax Reference​

Operators​

OperatorDescriptionExample
>Greater than{Temperature} > 80
<Less than{Pressure} < 10
>=Greater than or equal{Level} >= 100
<=Less than or equal{Flow} <= 5
==Equal to{Status} == 1
!=Not equal to{Mode} != 0
&&Logical AND{Temp} > 80 && {Pressure} > 50
||Logical OR{Alarm1} == 1 || {Alarm2} == 1

Common Alarm Patterns​

Alarm TypeExpressionDescription
High-High Limit{X} >= 200Critical high threshold
High Limit{X} >= 150 && {X} < 200Warning high threshold
Low Limit{X} > 5 && {X} <= 10Warning low threshold
Low-Low Limit{X} <= 5Critical low threshold
Range Violation{X} < 10 || {X} > 90Outside acceptable range
Equipment Fault{FaultCode} != 0Non-zero fault code
Communication Loss{Dev_PLC1_State} == falseDevice disconnected

Best Practices​

Alarm Design Principles​

Prioritize Alarms:

  • Use levels consistently (Critical = immediate action, Low = informational)
  • Limit Critical alarms to true emergencies (< 5% of total alarms)
  • Avoid alarm floods (too many alarms at once)

Clear Messaging:

  • Use descriptive alarm names: Oven_Temperature_High not Alarm_001
  • Write actionable messages: "Oven temperature exceeds 200°C - Check cooling system" not "High temp"
  • Include units in messages: "Pressure: 85 PSI (limit: 80 PSI)"

Avoid Nuisance Alarms:

  • Use alarm constraints to filter transient conditions
  • Set realistic thresholds based on normal operating ranges
  • Review alarm frequency and adjust thresholds if needed

Alarm Management Strategy​

Alarm Rationalization:

Alarm Performance Metrics:

MetricTargetAction if Exceeded
Alarm Rate< 6 alarms/hour/operatorReduce nuisance alarms
Standing Alarms< 5 active alarmsInvestigate root causes
Alarm Floods< 10 alarms/10 minutesAdd constraints, adjust priorities
Stale Alarms< 1% active > 24 hoursReview and resolve

Alarm Response Procedures​

Document Response Actions:

  • Create standard operating procedures (SOPs) for each alarm
  • Train operators on alarm priorities and responses
  • Use alarm messages to reference SOPs (e.g., "See SOP-123")

Alarm Acknowledgment:

  • Require operators to acknowledge alarms
  • Track acknowledgment times to measure response
  • Escalate unacknowledged critical alarms

Troubleshooting​

Alarm Not Triggering​

Cause: Trigger condition is incorrect or tag value doesn't meet the condition.

Solution:

  1. Check the trigger expression in Alarm Management
  2. Verify tag values in Tag Manager → Start Debug
  3. Test the expression manually:
    • If {Temperature} > 80, check if Temperature is actually > 80
  4. Check if the alarm has a constraint that hasn't been met yet

Alarm Triggering Too Frequently​

Cause: Threshold is too sensitive or sensor is noisy.

Solution:

  • Add an alarm constraint (e.g., 5-second delay)
  • Adjust the threshold to a more realistic value
  • Use a deadband in the expression: {Temperature} > 82 instead of {Temperature} > 80
  • Filter the tag value with a logic tag (moving average)

Alarm Not Appearing in Historical Table​

Cause: Alarm recording is disabled for this alarm.

Solution:

  1. Navigate to Alarm Management → Alarm Recording
  2. Verify the alarm is checked
  3. Click Save

Alarm Stuck in Real-time Table​

Cause: The trigger condition is still true, so the alarm hasn't recovered.

Solution:

  1. Check the current tag value
  2. Verify the trigger condition is no longer met
  3. If the condition should be false but the alarm persists, check for expression errors

Too Many Active Alarms (Alarm Flood)​

Cause: Multiple alarms triggered simultaneously due to a cascading failure.

Solution:

  • Identify the root cause alarm (usually the first to trigger)
  • Suppress downstream alarms that are consequences of the root cause
  • Use alarm groups to organize related alarms
  • Implement alarm shelving (temporarily disable non-critical alarms during emergencies)

Alarm Message Not Displaying Correctly​

Cause: Special characters or formatting issues in the message.

Solution:

  • Avoid special characters like <, >, & in alarm messages
  • Use plain text descriptions
  • Test the message in the alarm viewer

Next Steps​

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